Mass spectrometers often include at least one RF ion guide which is operated in a region of relatively high pressure where collisions occur between ions and background gas molecules. The ion kinetic energies may be arranged in some configurations so that such collisions are energetic enough to cause collision induced dissociation (CID) of ions. In other configurations, the collision energies may be relatively low so that such collisions primarily cause a reduction of ion kinetic energies, which is sometimes referred to ‘collision cooling’. Collision cooling is often used in addition to CID in the same ion guide.
One consequence of such collision cooling is that collision cooling can result in so much reduction of ions' kinetic energy that their motion through the ion guide becomes very slow or even stagnant. To alleviate this problem, RF ion guide configurations have been developed that incorporate a potential gradient along the ion guide axis, which provides a motive force to ensure that ions cooled by collisions continue their motion along the ion guide axis to the ion guide exit. In other configurations, ions can be directed to an RF ion guide exit end by such an axial field, but a local potential barrier at the exit end may be imposed in order to prevent the ions from exiting the ion guide until the potential barrier is lowered. Ions are then trapped and accumulated near the ion guide exit, and can be retained there while additional collision cooling occurs. At the opportune time, the trapped ions can be abruptly accelerated out the ion guide exit by switching voltages applied to the exit electrode and/or the ion guide electrodes so as to convert the potential barrier field to an acceleration field. Such trapping and collision cooling is advantageous, for example, to alleviate duty cycle limitations of orthogonal time-of-flight (TOF) analyzers, by retaining ions in the trap between TOF pulses. Trapping ions in this manner also allows them to be subjected to other manipulations, such as fragmentation by resonant excitation, or ion-ion interactions such as electron transfer dissociation (ETD).
Collision cooling with or without trapping also causes the width of the kinetic energy distribution of a population of ions within an ion guide to be reduced, that is, causes the kinetic energies of the ions to become more similar. Consequently, for example, some or all ions can be subsequently directed with the same nominal kinetic energy into an orthogonal acceleration TOF analyzer, or other mass spectrometer, thereby overcoming mass discrimination that would otherwise result from the disparate ion kinetic energies. Relatively broad ion kinetic energy distributions are exhibited, for example, in a broad mass-to-charge (m/z) distribution of fragment ions produced in a collision cell operated at relatively low gas pressures, where significant collision cooling does not occur. In this case, fragment ions tend to travel at about the same velocity as the precursor ion, so the ion kinetic energy distribution in the resulting population of fragment ions reflects the potentially broad m/z distribution in the ion population. Another example where the initial ion population exhibits a relatively broad ion kinetic energy distribution is the case where ions are introduced into a vacuum region from a higher-pressure region via a supersonic expansion of gas passing through the orifice between the two regions. In such a situation, ions of different m/z values end up with similar velocities, and therefore exhibit a wide ion kinetic distribution reflective of their wide m/z distribution. In all such situations, the incorporation of collision cooling in a high pressure ion guide region acts to narrow a broad ion kinetic energy distribution, and the addition of an axial field in such a high pressure region helps to maintain continuous motion of cooled ions toward the ion guide exit.
Alternatively, axial fields have been utilized in RF ion guides where the axial field is oriented to impede the motion of ions, essentially providing a repelling electric field that is adjusted to reject ions from an ion population that have less than some specified kinetic energy. This approach is used with advantage in some inductively coupled plasma mass spectrometry (ICP/MS) instruments to reduce or eliminate mass spectral interferences.
In still other configurations, RF ion guides having an axial field have been used in a high pressure vacuum stage of an atmospheric pressure ion source interface to a mass spectrometer, in order to improve ion transmission efficiency through to the subsequent lower pressure vacuum stage, while allowing the background gas to be pumped out.
A rectilinear quadrupole, having wide flat electrodes with widths of, for example, 82% of the separation between opposing electrodes, provides better ion transport properties than RF ion guides having round rods, especially when a collision gas is present. However, such an ion guide provides very little access via the spaces between the RF electrodes, which almost meet at the corners of the square electrode arrangement. Therefore, generating a significant axial field within such rectilinear ion guides is difficult.